The invention relates to an apparatus for reading a disc-shaped record carrier in which video signals are recorded in substantially spiral tracks in such a way that corresponding picture elements of consecutive pictures are situated at substantially the same circumferential position on said record carrier, an address code being contained in at least one of the two fields of each video picture in said video signals at a predetermined position in the field blanking after the field-synchronizing pulse, which apparatus comprises:
an optical system for aiming a light beam at the record carrier, said light beam being modulated by the record carrier, and for converting said modulated light beam into an electric signal, PA1 a converter for converting said electric signal into a radial tracking-error signal which corresponds to the radial position of the landing spot of the light beam relative to a desired track, PA1 positioning means for controlling the position of the landing spot of the light beam as a function of the radial tracking-error signal, PA1 a jump-signal generator for generating a control signal for causing the landing sport of the light beam to be shifted over a predetermined distance in response to a jump signal, and PA1 means for applying said control signal to the positioning means during said jump. PA1 means for generating a first signal which is a continuous measure of the duration of the period of time up to a reference point in the field-blanking, which point precedes said address code, PA1 means for generating a second signal under command of the jump signal, which second signal is a measure of the anticipated duration of the jump to be performed in response to said command, and PA1 means for continuously comparing said first and said second signal and, in relation to the instant at which the two durations corresponds, generating a start signal for the jump-signal generator at least if said instant appears prior to the field synchronizing pulse preceding said address code.
An example of such an apparatus is the video-disc player which is commercially available from N. V. Philips' Gloeilampenfabrieken under the designation "Laservision" and which is described inter alia in the magazine "Philips Technical Review", Vol. 33, 1973, No. 7 pages 177-193 and U.S. Pat. No. Re. 29,963. In this known apparatus a jump of one or a few tracks is made in order to obtain still picture, slow motion etc. In said apparatus a predetermined signal is applied via the field synchronizing pulse during the field-blanking interval in order to obtain a jump of the positioning means over a specific distance.
For some uses, such as in interactive video disc players which cooperate with a game computer, it is desirable to have the possibility of changing the scene being reproduced. Accordingly, the landing spot of the laser beam must be changed very rapidly over a large number of tracks in a radial direction by the positioning means, for example, a pivotal mirror, though it is also possible to use other actuators such as linearly moving actuators. In particular because in such a case a plurality of scenes may have been recorded in parallel and may change from track to track, it is essential that such a jump in a radial direction ends at the desired track, because otherwise a scene other than the desired scene is reached. In order to be able to ascertain immediately whether a jump is correct it is therefore necessary that the address code can be read after the jump and consequently the jump is always terminated before said address code is read.